1. Field of the Invention
This invention relates to a filter for collecting fine particles in exhaust gases discharged from combustion mechanisms such as diesel engines
2. Description of the Prior Art
The exhaust pipe of a diesel engine is provided with a purifier for purifying the exhaust gas by collecting fine particles, such as carbon particles, contained in the gas. FIG. 16 shows an example of such a purifier.
In the drawing, a collecting filter 1 is formed as a cylindrical body having a honeycomb structure, which consists of a large number of cells 11 separated from each other by cell partitions 12 (FIG. 17), with adjacent cells 11 being alternately closed at the upstream and downstream ends thereof. Exhaust gas, introduced into the filter 1 at the upstream end thereof, enters those cells 11 which are open on the upstream side, and passes through the porous sections of the cell partitions 12 to flow into the adjacent cells 11, from which it is discharged to the downstream side. In this process, the fine carbon particles contained in the exhaust gas are arrested by the cell partitions 12 and accumulated thereon.
As this accumulation of fine particles progresses, the air-flow resistance of the filter increases, resulting in an increase in the differential pressure across the filter 1. Since this will cause the engine output to be lowered, it is necessary to periodically remove the accumulated fine particles. The removal is effected by, for example, a heater 5 provided on the upstream-side end surface of the filter 1 and serving to burn the collected fine particles.
A problem with this purification method by burning is that it involves an excessive temperature rise in the collecting filter, in particular, in the central portion thereof. Such a temperature rise will cause a large temperature gradient between the central portion of the filter and the peripheral portion thereof, which is at a relatively low temperature, resulting in the filter being damaged by heat. Further, in the low-temperatured peripheral portion of the filter, it often happens that some of the accumulated particles remain unburned, thus preventing perfect purification.
This situation is illustrated in the graph of FIG. 18. In this graph, the solid line represents changes in the temperature with passage of time in the central portion (the portion indicated at 14 in FIG. 16) of the filter 1, and the broken line represents those in the peripheral filter portion (the portion indicated at 15 in FIG. 16). The maximum temperature T1 in the central filter portion can become so high as to damage the filter 1. Further, due to the large temperature difference .DELTA.T1 (approx. 300.degree. C.) between the central and peripheral portions, this temperature involves an excessive temperature gradient The relatively low temperature in the peripheral region is due to the fact that the heat in this region is easily dissipated to the exterior through the tube wall of the container 3 lodging the filter.
An attempt to solve the problem of temperature rise in the central region is disclosed in, for example, Japanese Utility Model Unexamined Publication No. 59-152119, according to which the thickness of the cell partitions in the central region of the filter is made larger than that of the cell partitions in the peripheral filter region, that is, a difference in the level of wall thickness is provided across a predetermined boundary section between the two regions, thereby attaining an increase in heat capacity and avoiding a rapid temperature rise. This arrangement, however, involves a large difference in heat capacity across the boundary section where the cell-partition thickness changes, thereby causing a difference in temperature. Thus, with this proposed design, heat damage is liable to be caused in the boundary section mentioned above.